Introduction

The Time Ephemerides software project consists of software and
associated data to create, manipulate, and interpolate time
ephemerides of the Earth (and potentially other solar system bodies).
These time ephemerides consist of convenient approximations to
relativistic time-dilation integrals which have been determined using
numerical quadrature of quantities supplied by JPL
ephemerides of the major bodies of the solar system. Time ephemerides
of the Earth help to determine general-relativistic corrections for
Earth-based clocks due to the time varying velocity relative to the
solar system barycenter and time-varying gravitational field
experienced by such clocks. Such corrections are fundamentally
important since they help determine the independent variable of the
JPL ephemerides, corrections to pulse-arrival times of pulsars,
determinations of precise radial velocities, interpretation of
spacecraft-ranging data, etc. For important background information on
time ephemeris calculations
see this
paper. For example, equation 3 of that paper defines the
relativistic integral approximated by the various time ephemerides of
the Earth.

Software Subprojects

The Time Ephemerides software is divided into the following
subprojects:

ephcom2

timeephem_generate

timeephem_interpolate

The ephcom2 software project is a
major revival of the ephcom1 software for
manipulating and interpolating ephemeris data in standard JPL
ascii and binary forms. The ephcom1 software was last
released under
the LGPL
by Paul Hardy in May 2004 at
the ephemeris.com website.
The ephcom2 software fixes several important bugs (and many
minor bugs) in ephcom1 and adds some useful additional
functionality (a convenient Fortran interface and a CMake-based
build and test system). The ephcom2 software is licensed
under
the LGPL.
For a more detailed look at ephcom2 please
see here.

The timeephem_generate software project contains software to
perform the necessary quadrature of the time-dilation integral
discussed above based on data supplied by a particular JPL ephemeris.
The result of these calculations is an ascii version of a time
ephemeris which is similar in form to the ascii form of the
corresponding JPL ephemeris.

The timeephem_interpolate software project contains software to
manipulate and interpolate the ascii time ephemeris data supplied by
the timeephem_generate project.

The timeephem_generate and timeephem_interpolate software have been
used previously to calculate the TE200 and TE405 time ephemerides
(corresponding to the JPL ephemerides DE200 and DE405) that were
discussed in the above paper. Both software packages are being
modernized (including a new CMake-based build and test system) and
will depend strongly on ephcom2. Both software packages should be
released in the forthcoming months under
the LGPL.